Methods and systems disclosed herein relate generally to enable long endurance Unmanned Underwater Vehicle (UUV) missions by providing the ability to use seafloor features of opportunity as ‘landmarks’ to reduce/constrain position error growth by providing relative position fix accuracies on the order of one meter.
Currently, underwater acoustic systems (Ultra-Short Base Line, Long Base Line, Beacons, U/W GPS, etc.) are expensive/prohibitive to deploy and limit the geographic area of travel of the UUV. Surfacing for GPS fix is hazardous and ineffective for deep water missions. Inertial and Doppler Velocity Log (DVL) navigation (dead-reckoning) is standard practice for high-grade UUVs, but has unbounded position error that grows with time. Multibeam (bathymetry) sonars have inadequate resolution to achieve the 1 m goal. Imaging sonars (Sidescan, Synthetic Aperture) have superior spatial resolution compared to other acoustic systems but historically have inadequate pixel position accuracy relative to vehicle. Standard optical image processing approaches are ineffective because spatial resolution is too poor, there is an insufficient number of pixels on small targets, the high grazing angle results in pixel size distortion with range (on the order of 20:1), and there is high noise due to the environment. Fully automated approaches for finding targets remain unsuccessful (missed targets, high false detect). Manual approaches are untenable (tedious, manual reviewing of imagery can take as long as the mission). Thus, all existing positioning methods are inadequate for long endurance submerged unmanned underwater vehicle missions.
The position error growth while dead-reckoning can be constrained by using detected targets along the vehicle's path. For example, if a vehicle sees a target at time t1 and the position error at that time is e1, when the vehicle sees the same target later at time t2 it will appear to be in a different position due to the accumulated vehicle position error. The vehicle's position can be adjusted to account for this difference and its position error set back to e1, the position error at the first sighting. While this concept of constraining position error is not new, the ability to accomplish it with acceptable accuracy using underwater sensors has not been done previously. It is a difficult problem to create vehicle position updates using seafloor objects (referred to herein interchangeably as ‘targets’) that may be seen, as few as only two times during a mission. The techniques can be applied to simpler situations where a target is seen many times during the same mission, and to the creation of a target database that can be used for position correction in subsequent missions.
To identify suitable targets, multibeam sonar systems have high position accuracy, but their spatial resolution is far too poor for doing bottom feature position estimates except with very large targets and thus yield inadequate position accuracy. What is needed is to be able to identify suitable targets using acoustic imagery systems (traditional sidescan, Synthetic Aperture Sonar) which are presently the only existing technologies suitable for the purpose of this invention. However, even though sidescan sonar technology has advanced considerably in the last two decades, it still lacks sufficient resolution. A suitable target may only have a few pixels on it. Additionally, it typically has significant pixel distortion with range (as much as 20:1) due to typically low grazing angles and often has high levels of noise due to the environment. Consequently, target identification methods used commonly with optical systems (that do not suffer these problems) to analyze the ‘image’ of a target cannot be readily used on acoustic imagery. The imagery can be scanned manually for targets, but this process is tedious and can take longer than the mission itself. Methods that address this problem through scene recognition (using many points within an image for position registration) are a topic of current research and do not yet work with sidescan imagery. What is needed is to combine target identification that relies on the use of small, isolated targets for initial detection and subsequent redetection with further new technology developments to reduce UUV fix position inaccuracies.
Historically, sidescan sonars were developed to be inexpensive systems that were towed behind a host vehicle, used merely to visually identify areas or objects of interest. Consequently, the imagery created had inherently poor position information and the desired pixel positioning accuracy (one meter with overall system error less than ten meters) could not be achieved. To overcome this deficiency, a tight and very precise coupling of a sidescan sonar with vessel high-quality navigation sensors can achieve improvements in pixel position accuracy. Improvements to achieve the coupling include, but are not limited to, time synchronization between the sonar and position/attitude measurement sensors, logging of the necessary data at a sufficient rate and resolution, data post-processing to apply linear and angular offsets, accounting for sonar height above the seafloor, and using sufficient numeric resolution.
What is needed is to enable long endurance UUV missions by providing the ability to use seafloor features of opportunity as ‘landmarks’ to reduce/constrain position error growth. A goal is to achieve relative position fix accuracies on the order of one meter.